The present invention relates to a spacer for use in testing information recorded discs, and particularly to a spacer for use in testing for effects of temperature and humidity on such discs.
A capacitive video disc system has been described by Clemens in U.S. Pat. No. 3,842,194. The disc described comprises a plastic disc containing an information track having audio, video and color information in the form of a surface relief pattern arranged in the surface of the disc which can be played back with a stylus. The video disc of Clemens had a conductive metal coating to provide the conductivity required for capacitive pickup and a thin layer of a dielectric material thereover. An electrode on the playback stylus completed the capacitor.
Improvements have been made in this system whereby the disc is made from a conductive plastic material, as disclosed by Fox et al in copending application Ser. No. 105,550 filed Dec. 20, 1979, which is a continuation of application Ser. No. 818,279 filed July 25, 1977, now abandoned. A polyvinylchloride based molding composition is mixed with sufficient amounts of finely divided carbon black particles so that the resulting composition has the conductivity required for capacitive playback. A thin layer of the polyvinylchloride surrounds each of the conductive carbon particles so that a thin dielectric layer is present at the surface. The use of a conductive plastic eliminates the need for separate metal and dielectric layers on the disc surface and thus has simplified manufacture of the disc.
If the disc described in the Fox et al application is not manufactured properly, heat and moisture can adversely affect the playing characteristics of the discs. In order to determine whether the discs are manufactured properly, sample discs are subjected to a "stress" test in which the discs are subjected to heat and high humidity and then tested for playing characteristics. For this test a plurality of the discs are stacked on a spindle with plastic spacer rings between adjacent discs to space the discs apart. The spindle of discs are placed in a chamber which is heated and filled with moisture to create a high humidity in the chamber. After subjecting the discs to heat and humidity for a predetermined period of time, the spindle of discs are removed from the chamber, the discs are removed from the spindle and then tested for playing characteristics. It has been found that although the discs themselves are dry when removed from the spindle, the plastic spacer rings tend to retain some water which either drips or splatters onto the discs as they are removed from the spindle. This provides large water spots on the discs which give false adverse playing characteristic readings.